After upgrading `pytest-asyncio` to the latest version, lots of tests
started failing in CI only on Python 3.9:
`RuntimeError: There is no current event loop in thread 'MainThread'`
Apparently these tests may have only been passing previously due to
issues in earlier versions of `pytest-asyncio`. Changing these tests to
async seems to fix the failures on Python 3.9.
Related issue:
https://github.com/pytest-dev/pytest-asyncio/issues/1039